The benefits of massage and acupressure have been known for centuries. There are many different kinds of massage styles in existence, and each style has some unique massage stroke. An important Swedish massage stroke is known as the friction or shear stroke. The friction stroke is the deepest of Swedish massage strokes. In applying a friction stroke, the masseur's hands move with the skin instead of gliding across the skin. There are many varieties of the friction stroke such as, for example, this stroke can be done parallel to the deeper muscle fibers (longitudinal friction), in a circular motion over the deeper muscle fibers (circular friction), or across the deeper muscle fibers (cross fiber or transverse friction). Cross fiber friction strokes, in particular, are believed to reduce fibrosis and encourage formation of strong, pliable scar tissue at a healing site of injuries. Cross fiber friction strokes are further believed to reduce crystalline roughness that forms between tendons and their sheaths that can result in painful tendonitis and to prevent or soften myofascial adhesions.
There are three modes of friction that can be utilized in a massage stroke: static friction; sliding friction; and rolling friction. Static friction is characteristic of acupressure and Shiatsu where static friction keeps the acupressure contact from sliding across the massage recipient's body but the primary form of therapy is a force normal to the user's body. Sliding friction is characteristic of Swedish massage where sliding friction provides a shear force applied to the recipient's body which is equal in importance to the normal force applied to the recipient's body. Rolling friction is rarely characteristic of massage by hand but is frequently used by automatic massage systems having massage rollers.
Due to static friction, a static friction force is produced which keeps an object, such as a massaging manipulator, at rest while an external force is applied to the massaging manipulator which is less than a static limit. The static friction force is a reaction force, that is, its value depends on the direction and magnitude of the applied external force trying to move the massaging manipulator along the recipient's skin. The static limit is determined by a coefficient of static friction μstatic multiplied by the normal component of the applied external force. As the applied external force increases, the massaging manipulator “breaks loose” and begins to move relative to the recipient's skin once the applied external force is above the static limit. The magnitude of the applied external force which overcomes the static friction force is called the starting force. It should be appreciated that, due to static friction, the manipulator can produce a shear force on the recipient prior to movement of the manipulator relative to the recipient.
Due to sliding friction (also referred to as dynamic or kinetic friction), a sliding friction force is produced which resists sliding movement of the manipulator when the static limit is overcome and the manipulator is sliding relative to the recipient's skin. The sliding friction force is less then the maximum static friction force but still has some gripping force on the recipient's skin. Typically, sliding friction produces heat as the manipulator is moved along the recipient's skin.
Rolling friction is a much more complex. Due to rolling friction, two primary forces are produced which oppose rolling motion of a massage roller, one acts at the roller's center and the other acts at the area of contact on the user's skin. The force at the roller's center is created by rubbing contact at an axle supporting the roller. The force at the contact zone is created by contact between the roller and the recipient and enables the roller to have traction so that the roller rolls along the recipient rather than slips Another important factor is rolling resistance. Rolling resistance is a loss due to roller/skin deformation or compression and not due to drag between the two surfaces. In rolling friction, the molecules on the wheel's circumference execute cycloidal motion so the molecules in the wheel's contact area hop along the ground as one molecule lifts off the ground another descends to replace it. Because rolling motion does not theoretically involve sliding, the coefficient of static friction applies in rolling friction as well as in static friction. This static friction is combined with the rolling resistance to create the total skin friction of the roller. This skin friction and the axle friction react with an applied force to allow rolling until a certain limit is reached. A state of skidding or slipping occurs when the limiting force is exceeded.
There is a multiplicity of massage devices known in the prior art. These devices range from handheld blocks or immobile probes to automatic massaging chairs and tables having massage rollers. A Backknobber or Jacknobber manufactured by Pressure Positive Company is an example of a typical immobile probe. The probe is used in a stationary fashion as with an acupressure stroke or in a sliding fashion across the recipient's back. When sliding, the probe generates a large amount of shear or friction force which is useful for cross fiber friction strokes as well as other types of strokes.
U.S. Pat. No. 6,283,928 discloses a typical massage apparatus incorporating rollers. The massage rollers are “roller-blade” type wheels that are mounted on a fixed bracket and engage a recipient. As the bracket translates up and down the user's back the rollers spin freely as they roll along the recipient's back. The free spinning or rolling motion of the rollers generate only a small amount of shear or friction force.
There are massaging roller-type devices which include braking mechanisms but the devices do not transfer a shear force to the user. For example, U.S. Pat. No. 6,213,962 discloses a massage device having two massage rollers mounted on a driven shaft which are rotatable relative to each other. Although the disclosure indicates that “braking means be provided for applying a frictional resistance against the rotation of [one of the rollers relative to the other]” and that a second brake means be provided to prevent the overall shaft from rotating when pressed by the user, both of these brakes do not allow the device to apply shear force to the user. In all embodiments of the device, the rollers are mounted behind a membrane which must be designed to slide freely relative to the rollers. In fact, the inventor notes that “it is preferred that the pair of right and left massaging rollers are coupled to the rotary shaft so as to be rotatable relative to the rotary shaft for preventing unnecessary friction against the affected part and the cover member.” See column 3, line 57.
There are also massaging devices with friction brakes which are fixed in the degree of friction which they apply. For example, see U.S. Pat. No. 6,071,253 which discloses a spinal column flexing fixture which has a roller with a friction brake. The brake applies a constant friction force which only applies a predetermined and fixed level of rolling resistance. The level of shear resistance cannot be adjusted without disassembling the device and replacing components. However, the roller does not have means of sensing or dynamically controlling shear forces applied to the user.
While each of these prior massaging devices may adequately perform a specific type of massage stroke or other manipulation, they are each limited in the types of massage strokes which they can perform. Accordingly, there is a need in the art for a massage device which is adjustable to selectively produce different magnitudes of shear force so that a larger number of different types of massage strokes can be produces by a single massage manipulator.